As used herein, the term “molten metal” means any metal or combination of metals in liquid form, such as aluminum, copper, iron, zinc and alloys thereof. The term “gas” means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, freon, and helium, that are released into molten metal. The components of the present invention are used in a molten metal bath, such as a molten aluminum bath, or comparable environment. A component according to the invention may be part of a device, such as a molten metal pump, scrap melter or degasser, or the component may not be part of a device.
Known molten-metal pumps include a pump base (also called a housing or casing), one or more inlets (an inlet being an opening in the housing to allow molten metal to enter a pump chamber), a pump chamber, which is an open area formed within the housing, and a discharge, which is a channel or conduit of any structure or type communicating with the pump chamber (in an axial pump the chamber and discharge may be the same structure or different areas of the same structure) leading from the pump chamber to an outlet, which is an opening formed in the exterior of the housing through which molten metal exits the pump casing. A rotor, also called an impeller, is mounted in the pump chamber and is connected to a drive system. The drive system is typically a rotor shaft connected to one end of a drive shaft, the other end of the drive shaft being connected to a motor. Often, the rotor shaft is comprised of graphite, the motor shaft is comprised of steel, and the two are connected by a coupling. As the motor turns the drive shaft, the drive shaft turns the rotor and the rotor pushes molten metal out of the pump chamber, through the discharge, out of the outlet and into the molten metal bath. Most molten metal pumps are gravity fed, wherein gravity forces molten metal through the inlet and into the pump chamber as the rotor pushes molten metal out of the pump chamber.
Molten metal pump casings and rotors usually employ a bearing system comprising ceramic rings wherein there are one or more rings on the rotor that align with rings in the pump chamber (such as rings at the inlet and outlet) when the rotor is placed in the pump chamber. The purpose of the bearing system is to reduce damage to the soft, graphite components, particularly the rotor and pump base, during pump operation. A known bearing system is described in U.S. Pat. No. 5,203,681 to Cooper, the disclosure of which is incorporated herein by reference. As discussed in U.S. Pat. Nos. 5,591,243 and 6,093,000, each to Cooper, the disclosures of which are incorporated herein by reference, bearing rings can cause various operational and shipping problems. To help alleviate this problem, U.S. Pat. No. 6,093,000 discloses a rigid coupling to enable the use of a monolithic rotor without any separate bearing member. The rigid coupling assists in maintaining the rotor centered within the pumping chamber and rotating concentrically (i.e., without wobble).
A number of submersible pumps used to pump molten metal (referred to herein as molten metal pumps) are known in the art. For example, U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 5,203,681 to Cooper, U.S. Pat. No. 6,093,000 to Cooper and U.S. Pat. No. 6,123,523 to Cooper all disclose molten metal pumps. The term submersible means that when the pump is in use its base is submerged in a bath of molten metal.
Three basic types of pumps for pumping molten metal, such as molten aluminum, are utilized: circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby generally equalizing the temperature of the molten metal. Most often, circulation pumps are used in a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of a reverbatory furnace to a different location such as a ladle or another furnace.
Gas-release pumps, such as gas-injection pumps, circulate molten metal while releasing a gas into the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium, from the molten metal. As is known by those skilled in the art, the removing of dissolved gas is known as “degassing” while the removal of magnesium is known as “demagging.” Gas-release pumps may be used for either of these purposes or for any other application for which it is desirable to introduce gas into molten metal. Gas-release pumps generally include a gas-transfer conduit having a first end that is connected to a gas source and a second submerged in the molten metal bath. Gas is introduced into the first end and is released from the second end into the molten metal. The gas may be released downstream of the pump chamber into either the pump discharge or a metal-transfer conduit extending from the discharge, or into a stream of molten metal exiting either the discharge or the metal-transfer conduit. Alternatively, gas may be released into the pump chamber or upstream of the pump chamber at a position where it enters the pump chamber.
Generally, a degasser (also called a rotary degasser) includes (1) a rotor shaft having a first end, a second end and a passage for transferring gas, (2) an impeller, and (3) a drive source for rotating the rotor shaft and the impeller. The first end of the rotor shaft is connected to the drive source and to a gas source and the second end is connected to the connector of the impeller. Examples of rotary degassers are disclosed in U.S. Pat. No. 4,898,367 entitled “Dispersing Gas Into Molten Metal,” U.S. Pat. No. 5,678,807 entitled “Rotary Degassers,” and U.S. application Ser. No. 09/569,461 to Cooper entitled “Molten Metal Degassing Device,” filed May 12, 2000, the respective disclosures of which are incorporated herein by reference.
Generally a scrap melter includes an impeller affixed to an end of a drive shaft, and a drive source attached to the other end of the drive shaft for rotating the shaft and the impeller. The movement of the impeller draws molten metal and scrap metal downward into the molten metal bath in order to melt the scrap. A circulation pump is preferably used in conjunction with the scrap melter to circulate the molten metal in order to maintain a relatively constant temperature within the molten metal. Scrap melters are disclosed in U.S. Pat. No. 4,598,899, to Cooper U.S. patent application Ser. No. 09/649,190 to Cooper, filed Aug. 28, 2000, and U.S. Pat. No. 4,930,986 to Cooper, the respective disclosures of which are incorporated herein by reference.
Molten metal pumps, scrap melters and degassers each have components that contact the molten metal bath while the device is in use. For example, the components of a molten metal pump that usually contact the molten metal bath while the pump is in use include: (a) the housing and all structures included on or in the housing, (b) the rotor, (c) the rotor shaft, (d) the support posts, (e) the gas-transfer conduit (if used), and (f) the metal-transfer conduit (if used). The components of a scrap melter or degasser that usually contact the molten metal while the device is in use include: (g) the rotor, and (h) the rotor shaft. There are also other components, such as temperature probes and lances, that are used in molten metal baths but that are not part of a larger device or assembly.
The materials forming the components that contact the molten metal bath should remain relatively stable in the bath. Structural refractory materials, such as graphite or ceramics, that are resistant to disintegration by corrosive attack from the molten metal may be used. As used herein “ceramics” or “ceramic” refers to any oxidized metal (including silicon) or carbon-based material, excluding graphite, capable of being used in the environment of a molten metal bath. “Graphite” means any type of graphite, whether or not chemically treated. Graphite is particularly suitable for being formed into pump components because it is (a) soft and relatively easy to machine, (b) not as brittle as ceramics and less prone to breakage, and (c) less expensive than ceramics.
Components comprised of graphite are still subject to corrosive attacks from the molten metal. Corrosion is usually more significant at the surface of the molten metal bath where oxygen and the molten metal interact causing oxidation and corrosion (the wearing away) of the graphite components. It has been known to place a protective coating on a graphite component by rubbing or otherwise applying cement to the component, sliding a ceramic (such as silicon carbide) sleeve onto the component (with the wet cement being between the sleeve and the component), and allowing the cement to dry thus adhering the sleeve to the component. It is also known to apply a ceramic sleeve to a component and to then insert cement at the top of the sleeve between the component and the sleeve to adhere the sleeve to the component. Some problems with these methods of adding a sleeve to a component are (a) the cement is sometimes unevenly applied, one reason for this being that the non-coated component is sometimes not centered in the sleeve, and (b) the sliding operation can scrape away some of the cement. Either of these factors, or others, may cause voids or air pockets in the dried cement between the non-coated component and the ceramic sleeve. Air pockets can lead to early failure of the component including the sleeve. Additionally, the thickness of the cement may simply be uneven, which can lead to component failure.
For example, molten metal can work its way into the air pockets and corrode the graphite behind the ceramic. Further, the air pockets provide no structural support for the sleeve. If something strikes the ceramic sleeve where there is an air pocket, the sleeve may break. Also, the air in the pocket expands while the component is in the molten metal bath, which may cause the cement to separate from the component or sleeve exacerbating the aforementioned problems. Additionally, the known methods of adding a sleeve to a component are time consuming, messy and may lead to a waste of cement.